CVI followed by coal tar pitch densification by VPI

ABSTRACT

Method for manufacturing pitch-densified carbon-carbon composite brake discs from carbon fiber preforms, by the following sequential steps: (a) providing a carbon-carbon composite brake disc preform; (b) heat treating the preform; (c) subjecting the heat-treated preform to Chemical Vapor Deposition/Chemical Vapor Infiltration processing; (d) infiltrating the preform with an isotropic low to medium char-yield pitch by Vacuum Pitch Infiltration processing or Resin Transfer Molding processing; (e) carbonizing the pitch-infiltrated preform; (f) machining the surfaces of the resulting carbonized preform; and (g) repeating steps (d) through (f) until the density of the carbon-carbon composite preform is at least 1.70 g/cc. The use of VPI equipment with isotropic, low to medium char-yield pitches for all densification steps following an initial CVD densification reduces capital and pitch materials cost. However, one or more RTM processing steps employing low to medium char-yield pitches may also be used to obtain improved economics.

FIELD OF THE INVENTION

This invention relates to the manufacture of carbon-carbon compositebrake discs from carbon fiber preforms which are densified with pitch.

BACKGROUND OF THE INVENTION

Chemical Vapor Deposition/Chemical Vapor Infiltration (CVD/CVI)processing entails a high capital investment cost. Combining ResinTransfer Molding (RTM) processing of mesophase or other high char-yieldpitches with CVD/CVI processing further increases capital costs. RTMprocessing also entails significant raw material costs, due to theexpensive nature of the high char-yield pitches typically used in RTMprocessing.

U.S. Pat. No. 5,837,081 (Ting et al.) discloses a method for making acarbon-carbon composite that includes densification of the preform usingpitch infiltration which may also include a combination of CVD withpitch infiltration. See e.g. lines 55-65 in column 4 of the Ting et al.patent.

U.S. Pat. No. 6,323,160 B1 (Murdie et al.) discloses a carbon-carboncomposite made from densified carbon foam which includes CVD/CVIprocessing and VPI pitch infiltration.

US 2006/0244165 A1 (Huang) discloses manufacturing carbon fiberreinforced ceramics as brake discs. The Huang process involvesdensification steps including CVD/CVI as well as pitch infiltration inwhich the preform is placed into a vacuum chamber with pitch introducedinto the chamber (i.e., VPI).

SUMMARY OF THE INVENTION

This invention utilizes a precise sequence of process steps to reducethe capital and material costs that are associated with pitchdensification of mesophase (high char-yield) pitches into carbon-carboncomposites by RTM as well as with densification that employs onlyCVD/CVI. More specifically, the present invention employs Vacuum PitchInfiltration (VPI) and/or RTM processing to densify the carbon-carboncomposites with isotropic (low to medium char-yield) pitches obtainedfrom coal tar, petroleum, or synthetic feedstocks. VPI processingentails lower capital costs than does CVD/CVI processing or RTM.Moreover, the use of the low cost ($0.20-$0.50/lb) isotropic, low tomedium char-yield pitches derived from coal tar, petroleum, or syntheticfeedstocks instead of the more expensive mesophase pitches($5.00-$10.00/lb) that are typically used for RTM processing helps tosignificantly reduce the cost of the final product.

A preferred embodiment of this invention, in terms of reduced capitaland pitch materials cost, will include the use of VPI equipment withisotropic (low to medium char-yield) pitches for all densification stepsfollowing an initial CVD densification. However, one or more RTMprocessing steps employing low to medium char-yield pitches may also beused to obtain improved economics.

Starting with a previously prepared nonwoven or random fiber-basedcarbon-carbon composite in the shape of a brake disc, this inventionemploys the following sequential steps:

-   (b) heat treating the carbon fiber preform at 1600-2540° C.;-   (c) subjecting the preform to a single cycle of CVD/CVI processing    to impart to the preform a density of about 1.1 g/cc to 1.4 g/cc;-   (d) infiltrating the preform with an isotropic low to medium    char-yield pitch (coal tar-derived, petroleum-derived, or    synthetic-derived) employing VPI or RTM processing, to impart to the    preform a density of about 1.3 g/cc to 1.6 g/cc;-   (e1) optionally stabilizing the pitch-infiltrated preform by heating    it in air at about 180° C.;-   (e2) carbonizing the pitch-infiltrated preform at 900-2400° C., e.g.    with a 4 hour hold to ensure uniform temperature in the furnace,    thereby converting the isotropic pitch to anisotropic    graphitic/graphitizable carbon;-   (e3) optionally heat treating the carbonized preform at 1600-2400°    C., with a 4 hour hold, to further graphitize the pitch matrix;-   (f) machining the surfaces of the carbonized pitch-infiltrated    preform; and-   (g) repeating steps (d) through (g) until the density of the preform    is at least 1.70 g/cc.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the invention will be betterunderstood after a reading and understanding of the below detaileddescription together with the following drawings wherein:

FIG. 1 is a flowchart depicting steps in the process of the presentinvention.

FIG. 2 is a flowchart depicting steps in the process of the presentinvention, including an optional stabilization step and an optional heattreatment step.

DETAILED DESCRIPTION OF THE INVENTION

The present invention makes use of processing modules which are known inthemselves. The advantages provided by the present invention lie in theselection and ordering of known processing modules to improve uniformityin the microstructure of the composite brake discs prepared inaccordance with this invention.

In one embodiment, this invention provides a method for the manufactureof pitch-densified carbon-carbon composite brake discs from carbon fiberpreforms by way of the sequential steps (a) through (g) described below.

Step (a) involves providing a carbon-carbon composite preform in theshape of a brake disc to be manufactured. Step (b) involves heattreating that preform at 1600-2540° C. Step (c) involves subjecting theheat-treated preform to CVD/CVI processing. The carbonized fiber preformproduced in CVD/CVI densification step (c) may: (i.) have a roughlaminar microstructure and be densified (e.g., at a temperature of 1275°C., a pressure of 210 Torr, and a C/H ratio of 1/4) to a density ofabout 1.4 grams per cubic centimeter; (ii.) have a smooth laminarmicrostructure and be densified (e.g., at a temperature of 1200° C., apressure of 630 Torr, and a C/H ratio of 1/4) to a density of about 1.2grams per cubic centimeter; or (iii.) have an isotropic microstructureand be densified (e.g., at a temperature of 1425° C., a pressure of 630Torr, and a C/H ratio of 1/4) to a density of about 1.1 grams per cubiccentimeter.

Step (d) involves infiltrating the preform with an isotropic low tomedium char-yield pitch, employing Vacuum Pitch Infiltration processingor Resin Transfer Molding processing. Those processing techniques arewell known in general. They are described in more detail hereinbelow.The isotropic pitch used in step (d) will normally be a low cost, low tomedium char-yield material derived from coal tar, petroleum, orsynthetic feedstock.

Step (e) involves carbonizing the pitch-infiltrated preform at 900-2400°C. In preferred embodiments, carbonization may be carried out at atemperature between about 1200 and 2000° C. in an inert nitrogenatmosphere or in a vacuum. Step (e) may optionally be directly precedingby a step comprising stabilizing the pitch-infiltrated preform byheating it in air at a temperature between about 150 and 250° C.(conveniently, for instance, at about 180° C.) to prevent pitchexudation. Instead of this stabilization step, pressure and a can may beused to prevent and contain any pitch exudation during heat treatment.Step (e) may optionally be directly followed by a step comprising heattreating the carbonized preform at 1600-2400° C., with a 4 hour hold, tofurther graphitize the pitch matrix. All, both, or none of theseoptional steps may be employed in accordance with various embodiments ofthe present invention.

Step (f) involves machining the surfaces of the carbonized preformresulting from the preceding steps. The purpose of intermediatemachining between densification processing steps is to grind down thesurfaces of the annular discs in order to expose porosity in thesurfaces of the discs, thereby facilitating further densification.

Final “step” (g) involves repeating steps (d) through (f) until thedensity of the carbon-carbon composite preform is at least 1.70 g/cc. Ina first iteration of step (d), for instance, the carbon-carbon compositebrake disc preform may be densified with an isotropic pitch to a densityof approximately 1.3-1.6 grams per cubic centimeter. In a thirditeration of step (d), for instance, the carbon-carbon composite brakedisc preform is densified with coal tar pitch to a density ofapproximately 1.75 grams per cubic centimeter. Persons skilled in theart will readily identify the number of iterations of VPI (andoptionally RTM) processing with low to medium char-yield pitch that willbe appropriate to reach target densities in the carbon-carbon compositebrake discs being manufactured.

Another embodiment of the present invention is a method for themanufacture of pitch-densified carbon-carbon composite brake discs fromcarbon fiber preforms, which method comprises sequentially: (a)providing a carbon-carbon composite preform in the shape of a brakedisc; (b) heat treating said preform at 1600-2540° C.; (c) subjectingthe heat-treated preform to CVD/CVI processing; (d) infiltrating thepreform with an isotropic low to medium char-yield pitch, employingVacuum Pitch Infiltration processing or Resin Transfer Moldingprocessing; (e) carbonizing the pitch-infiltrated preform at 900-2400°C.; (f) machining the surfaces of the resulting carbonized preform; and(g) repeating steps (d) through (f) until the density of thecarbon-carbon composite preform is at least 1.70 g/cc, provided that inat least one iteration of step (d), the isotropic low to mediumchar-yield pitch is replaced by a mesophase pitch with a high charyield. In a further variation of this embodiment, at least one iterationof step (d) employs Resin Transfer Molding processing.

By its utilization of the above-described material choices andprocessing sequences, the present invention thereby avoids variation inthe friction and wear performance of the brake discs and improves theirstrength, oxidation resistance, etc. This invention also improves theeconomics of disc manufacture. Various “modules” that may be used inaccordance with the present invention are summarized below.

Heat Treatment

Intermediate and/or final heat treatment of the preforms is usuallyapplied to modify the crystal structure and order of the carbon. Heattreatment is employed to modify the mechanical, thermal, and chemicalproperties of the carbon in the preform. Heat treatment of the preformsmay be conducted in the range of 1600° to 2450° C. The effect of such atreatment on graphitizable materials is well known. Higher temperaturesincrease the degree of order in the material, as measured by suchanalytical techniques as X-ray diffraction or Raman spectroscopy. Highertemperatures also increase the thermal conductivity of the carbon in theproducts, as well as the elastic modulus.

CVD/CVI

Chemical vapor deposition (CVD) of carbon is also known as chemicalvapor infiltration (CVI). In a CVD/CVI process, carbonized, andoptionally heat treated, preforms are heated in a retort under the coverof inert gas, typically at a pressure below 100 torr. When the partsreach a temperature of 900° to 1200° C., the inert gas is replaced witha carbon-bearing gas such as methane, ethane, propane, butane,propylene, or acetylene, or combinations of these gases. When thehydrocarbon gas mixture flows around and through the porous structures,a complex set of dehydrogenation, condensation, and polymerizationreactions occur, thereby depositing the carbon atoms within the interiorand onto the surface of the porous structures. Over time, as more andmore of the carbon atoms are deposited onto the structures, the porousstructures become more dense. This process is sometimes referred to asdensification, because the open spaces in the porous structures areeventually filled with a carbon matrix until generally solid carbonparts are formed. Depending upon the pressure, temperature, and gascomposition, the crystallographic structure and order of the depositedcarbon can be controlled, yielding anything from an isotropic carbon toa highly anisotropic, ordered carbon. US 2006/0046059 A1 (Arico et al.),the disclosure of which is incorporated herein by reference, provides anoverview of CVD/CVI processing.

VPI

Vacuum Pressure Infiltration (“VPI”) is a well known method forimpregnating a resin or pitch into a preform. The preform is heatedunder inert conditions to well above the melting point of theimpregnating pitch. Then, the gas in the pores is removed by evacuatingthe preform. Finally, molten pitch is allowed to infiltrate the part, asthe overall pressure is returned to one atmosphere or above. In the VPIprocess a volume of resin or pitch is melted in one vessel while theporous preforms are contained in a second vessel under vacuum. Themolten resin or pitch is transferred from vessel one into the porouspreforms contained in the second vessel using a combination of vacuumand pressure. The VPI process typically employs resin and pitches whichpossess low to medium viscosity. Such pitches provide lower carbonyields than do mesophase pitches. Accordingly, at least one additionalcycle of pitch infiltration of low or medium char-yield pitch (with VPIor RTM processing) is usually required to achieve a final density of 1.7g/cc or higher.

Carbonization

The carbonization process is generally well known to those skilled inthe art. The CVD/resin/pitch-infiltrated fiber preforms are heated in aretort under inert or reducing conditions to remove the non-carbonconstituents (hydrogen, nitrogen, oxygen, etc.) from the fibers andmatrix carbons. This process may be performed, for instance, by buryingthe foam preforms in a bed of activated carbon, enclosed in a superalloyretort with a sand seal. Carbonization of the infiltrated pitch can becarried out either in a furnace, a hot isostatic press, an autoclave, orin a uniaxial hot press. In each of these techniques, the impregnatedpart is heated to the range of 600° to about 1000° C., while maintainingan inert atmosphere in the pressure range of 1 to 1000 atmospheres. Inone approach, for instance, the retort is purged gently with nitrogenfor approximately 1 hour, then it is heated to 900° C. in 10-20 hours,and thence to 1050° C. in 1-2 hours. The retort is held at 1050° C. for3-6 hours, then allowed to cool overnight. Carbonization can be carriedout up to 1800° C. The higher the pressure, the higher the carbon yieldachieved, although the biggest gains in carbon yield are achieved atmoderate pressures up to 5000 psi.

Machining the Surfaces of the Preform

Standard machining processes, well known to persons skilled in the artof manufacturing carbon-carbon composite brake discs, are used in themanufacture of the carbon-carbon composite friction discs provided bythe present invention. Between densification processing steps, thesurfaces of the annular discs are ground down to expose porosity in thesurfaces. Once the final density is achieved, the annular discs areground to their final thickness using standard grinding equipment toobtain parallel flat surfaces, and then the inside diameter and outsidediameter regions are machined, typically using a CNC (computer numericalcontrol) Mill to provide the final geometry including such features asrivet holes and drive lugs.

EXAMPLES

The following non-limiting examples illustrate various options forimplementing the novel manufacturing method provided by the presentinvention.

Pitch CVD Infiltration Stabili- Pitch Pitch Preform Infiltration 1zation Infiltration 2 Infiltration 3 Nonwoven Rough VPI - YES VPI -VPI - carbon Laminar Isotropic- Isotropic- Isotropic- fiber MediumMedium Medium char yield char yield char yield Nonwoven Rough VPI - NORTM - VPI - carbon Laminar Isotropic- Mesophase- Isotropic- fiber MediumHigh Char Medium char yield yield char yield Nonwoven Smooth VPI - NOVPI - RTM - carbon Laminar Isotropic- Mesophase Isotropic fiber MediumHigh Char medium char char yield yield yield Nonwoven Smooth RTM - YESVPI - VPI - carbon Laminar Isotropic- Mesophase- Isotropic fiber Mediumhigh char Medium char yield yield char yield Nonwoven Isotropic VPI -YES VPI - VPI - carbon Isotropic- Isotropic Mesophase- fiber MediumMedium high char char yield char yield yield Nonwoven Isotropic RTM -YES VPI - VPI - carbon Isotropic- Mesophase- Isotropic fiber Medium highchar Medium char yield yield char yield

INDUSTRIAL APPLICABILITY

In terms of friction and wear performance, densifying a carbon compositepreform using multiple cycles of CVD results in uniform carbon matrixmicrostructures throughout the thickness of the composite. This uniformmicrostructure comes at a cost, due to the high capitalization costs forCVD furnaces. In the present invention, only a single CVD cycle is usedto reproduce a uniform microstructure at the fiber matrix interfacesthroughout the thickness of the composite. No subsequent CVD/CVIdensification cycles are necessary. In place of subsequent CVD cycles,in this invention the carbon composite preform is further densified withlow to medium char-yield isotropic pitch, using VPI or RTM equipment, toprovide a more economical method of achieving a final density of 1.7g/cc or higher.

In terms of manufacturing economics, the hybrid composite concept (CVDand pitch matrices) embodied in the present invention enables the use oflow cost pitch materials combined with low cost capitalization forprocessing equipment (VPI and RTM) which produces carbon frictionmaterials with consistent properties and friction and wear performance.

The present invention has been described herein in terms of illustrativeembodiments. Modifications and additions to these embodiments will beapparent to those skilled in the art upon a reading of the foregoingdescription. All such modifications and additions form a part of thepresent invention to the extent they fall within the scope of theseveral claims appended hereto.

1. A method for the manufacture of pitch-densified carbon-carboncomposite brake discs from carbon fiber preforms, the method comprisingthe following sequential steps: (a) providing a carbon-carbon compositepreform made from nonwoven carbon fiber fabric, said preform beingconfigured in the shape of a brake disc; (b) heat treating said preformat 1600-2540° C.; (c) directly following step (b), subjecting theheat-treated preform to Chemical Vapor Deposition/Chemical VaporInfiltration processing to produce a carbonized fiber preform which:(i.) has a rough laminar microstructure and is densified to a density ofabout 1.4 grams per cubic centimeter; or (ii.) has a smooth laminarmicrostructure and is densified to a density of about 1.2 grams percubic centimeter; or (iii.) has an isotropic microstructure and isdensified to a density of about 1.1 grams per cubic centimeter; (d)infiltrating the preform with an isotropic low to medium char-yieldpitch derived from coal tar, employing Vacuum Pitch Infiltrationprocessing; (e) stabilizing the pitch-infiltrated preform by heating itin air at about 180° C. and then carbonizing the pitch-infiltratedpreform at 900-2400° C.; (f) machining the surfaces of the resultingcarbonized preform; and (g) repeating steps (d) through (f) at least twoadditional times, to raise the density of the carbon-carbon compositepreform to at least approximately 1.75 g/cc, wherein the carbon-carboncomposite brake disc preform is densified with an isotropic coal tarpitch to a density of approximately 1.3-1.6 grams per cubic centimeterin the first implementation of step (d) and wherein the carbon-carboncomposite brake disc is densified with coal tar pitch to a density of atleast approximately 1.75 grams per cubic centimeter in the thirdimplementation of step (d).
 2. The method of claim 1, wherein step (e)is directly followed by a step comprising heat treating the carbonizedpreform at 1600-2400° C., with a 4 hour hold, to further graphitize thepitch matrix.
 3. The method of claim 1, where the isotropic pitch usedin step (d) is a low to medium char-yield material derived from coaltar.
 4. The method of claim 1, wherein the carbonized fiber preformproduced in CVD/CVI densification step (c): (i.) has a rough laminarmicrostructure and is densified at a temperature of 1275° C., a pressureof 210 Torr, and a C/H ratio of 1/4; (ii.) has a smooth laminarmicrostructure and is densified at a temperature of 1200° C., a pressureof 630 Torr, and a C/H ratio of 1/4; or (iii.) has an isotropicmicrostructure and is densified at a temperature of 1425° C., a pressureof 630 Torr, and a C/H ratio of 1/4.
 5. The method of claim 1, whereincarbonization step (e) is carried out at a temperature between about1200 and 2000° C. in an inert nitrogen atmosphere or in a vacuum.
 6. Amethod for the manufacture of pitch-densified carbon-carbon compositebrake discs from carbon preforms, the method comprising the followingsequential steps: (a) providing a carbon-carbon composite preform madefrom nonwoven carbon fiber fabric, said preform being configured in theshape of a brake disc; (b) heat treating said preform at 1600-2540° C.;(c) directly following heat treating step (b), subjecting theheat-treated preform to Chemical Vapor Deposition/Chemical VaporInfiltration processing to produce a carbonized fiber preform which;(i.) has a rough laminar microstructure and is densified to a density ofabout 1.4 grams per cubic centimeter; or (ii.) has a smooth laminarmicrostructure and is densified to a density of about 1.2 grams percubic centimeter, or (iii.) has an isotropic microstructure and isdensified to a density of about 1.1 grams per cubic centimeter; (d)infiltrating the preform with an isotropic low to medium char-yieldpitch derived from coal tar, employing Vacuum Pitch Infiltrationprocessing; (e) conducting oxidative stabilization at a temperaturebetween about 150 and 250° C. to prevent pitch exudation and thencarbonizing the pitch-infiltrated preform at 900-2400° C.; (f) machiningthe surfaces of the resulting carbonized preform; and (g) repeatingsteps (d) through (f) at least two additional times, to raise thedensity of the carbon-carbon composite preform to at least approximately1.75 g/cc, wherein the carbon-carbon composite brake disc preform isdensified with an isotropic coal tar pitch to a density of approximately1.3-1.6 grams per cubic centimeter in the first implementation of step(d) and wherein the carbon-carbon composite brake disc preform isdensified with coal tar pitch to a density of at least approximately1.75 grams per cubic centimeter in the third implementation of step (d).